Person为自定义的类
public class Person {
private String name;
private int age;
public Person() {
}
public Person(String name, int age) {
this.age = age;
this.name = name;
}
@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", age=" + age +
'}';
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
key:String类型
value:Person类
private static void show01(){
//创建HashMap集合
HashMap<String,Person> map = new HashMap<>();
//添加元素
map.put("斗罗大陆",new Person("唐家三少",30));
map.put("三生三世",new Person("唐家三少",30));
map.put("哈利波特",new Person("J.K.罗琳",27));
map.put("朝花夕拾",new Person("周树人",66));
map.put("朝花夕拾",new Person("鲁迅",37));
Set<String> set = map.keySet();
for (String key : set){
Person person = map.get(key);
System.out.println(key+"----"+person);
}
}
结果为:
此时,主键“朝花夕拾”只显示了一次
ps:Person中toString方法没写时,无法输出为字符串
key:Person类
value:String类型
private static void show02(){
//创建HashMap集合
HashMap<Person,String> map = new HashMap<>();
//添加元素
map.put(new Person("唐家三少",30),"斗罗大陆");
map.put(new Person("唐家三少",30),"三生三世");
map.put(new Person("J.K.罗琳",27),"哈利波特");
map.put(new Person("周树人",66),"朝花夕拾");
map.put(new Person("鲁迅",37),"朝花夕拾");
/*Set<Person> set = map.keySet();
for (Person key : set){
String s= map.get(key);
System.out.println(key+"----"+s);
}*/
Set<Map.Entry<Person,String>> set = map.entrySet();
for (Map.Entry<Person,String> entry : set){
Person person = entry.getKey();
String s =entry.getValue();
System.out.println(person+"----"+s);
}
}
显示:
此时主键“唐家三少”不唯一
拍黑板!!!!!
给Person类重写hashCode()方法和equals()方法,保证key唯一!
非自定义类型,不重写该方法,也可以保证key唯一!
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
Person person = (Person) o;
return age == person.age &&
Objects.equals(name, person.name);
}
@Override
public int hashCode() {
return Objects.hash(name, age);
}
结果为:
主键唯一了!